This grant proposal focuses on the role of a basic-helix-loop-helix transcription factor, human achaete-scute homolog-1 (hASH1) in regulating the phenotype of the most aggressive and highly metastatic of human lung tumors, small cell lung carcinoma (SCLC). hASH1 is a human counterpart to the Drosophila achaete-scute complex (AS-C), a conserved family of transcription factors that are essential for the commitment of primitive neural precursor cells. The investigator has determined that hASH1 is expressed in lung cancers in a pattern tightly correlated with the presence of the neuroendocrine phenotype including classic SCLC cells. Genetic manipulations which mimic a transition between SCLC and non-SCLC phenotypes resulted in extinction of hASH1 expression. Dr. Ball has also demonstrated expression of the gene in normal fetal pulmonary endocrine cells, clustered in neuroepithelial bodies. Since pulmonary endocrine cells are among the first differentiated cell types to appear in development of the pulmonary airway epithelium, this expression pattern places hASH1 in a position to mediate essential processes in both lung development and establishment of the SCLC phenotype. A hallmark of both fetal pulmonary endocrine cells and SCLC is the growth of cells in clustered, tightly organized, anchorage-independent aggregates. In Drosophila nervous system development, expression of the AS-C triggers clustering events and cell-cell signaling which results in emergence of a single dominant neuroblast and down-regulation of the AS-C in adjoining cells, which then assume an epithelial fate. Many of the signaling molecules and transcription factors that participate in this lateral inhibition process are now known to have mammalian homologs; a striking degree of functional homology is also now emerging. In a series of transfections experiments, they have shown that exogenous expression of hASH1 in lung cancer cells which do not endogenously express this gene can induce growth of cells in dense floating aggregates in reduced serum. Such findings indicate that hASH1 may be critical to the SCLC phenotype, including its characteristic aggregative behavior. The proposed series of studies now seek to determine: 1) features of lung cancer neuroendocrine differentiation and growth, particularly related to the function of paracrine growth factors, that may be modulated by hASH1; 2) the molecular events and changes in surface adhesion molecules that may underlie hASH1 mediated cell-cell adhesion in lung cancer; 3) the pathways regulating hASH1 gene expression in lung cancer, with an emphasis on evolutionary conserved factors from Drosophila neurogenesis; 4) the role of hASH1 expression in pulmonary endocrine cells in normal lung development and in response to experimental lung injury. These studies will provide a powerful approach to understanding the origins of SCLC and the differentiation relationships that link this tumor to the other major forms of this disease, potentially leading to the discovery of novel therapeutic targets in lung cancer.